This invention relates to an age-hardenable, martensitic steel alloy, and in particular to such an alloy and an article made therefrom in which the elements are closely controlled to provide a unique combination of high tensile strength, high fracture toughness and good resistance to stress corrosion cracking in a marine environment.
Heretofore, an alloy designated as 300M has been used in structural components requiring high strength and light weight. The 300M alloy has the following composition in weight percent:
______________________________________ wt. % ______________________________________ C 0.40-0.46 Mn 0.65-0.90 Si 1.45-1.80 Cr 0.70-0.95 Ni 1.65-2.00 Mo 0.30-0.45 V 0.05 min. ______________________________________
and the balance is essentially iron. The 300M alloy is capable of providing tensile strength in the range of 280-300 ksi.
A need has arisen for a high strength alloy such as 300M but having high fracture toughness as represented by a stress intensity factor, K.sub.IC,.gtoreq.100 ksi .sqroot.in. The fracture toughness provided by the 300M alloy, represented by a K.sub.IC of about 55-60 ksi .sqroot.in, is not sufficient to meet that requirement. Higher fracture toughness is desirable for better reliability in components and because it permits non-destructive inspection of a structural component for flaws that can result in catastrophic failure.
An alloy designated as AF1410 is known to provide good fracture toughness as represented by K.sub.IC .gtoreq.100 ksi .sqroot.in. The AF1410 alloy is described in U.S. Pat. No. 4,076,525 ('525) issued to Little et al. on Feb. 28, 1978. The AF1410 alloy has the following composition in weight percent, as set forth in the '525 patent:
______________________________________ wt. % ______________________________________ C 0.12-0.17 Mn .05-.20 S 0.005 max. Cr 1.8-3.2 Ni 9.5-10.5 Mo 0.9-1.35 Co 11.5-14.5 REM 0.01 max. ______________________________________ REM = rare earth metals
and the balance is essentially iron. The AF1410 alloy, however, leaves much to be desired with regard to tensile strength. It is capable of providing ultimate tensile strength up to 270 ksi, a level of strength not suitable for highly stressed structural components in which the very high strength to weight ratio provided by 300M is required. It would be very desirable to have an alloy which provides the good fracture toughness of the AF1410 alloy in addition to the high tensile strength provided by the 300M alloy.